Organic light emitting devices ("OLEDs") have been known for approximately two decades. All OLEDs work on the same general principles. One or more layers of semiconducting organic material is sandwiched between two electrodes. An electric current is applied to the device, causing negatively charged electrons to move into the organic material(s) from the cathode. Positive charges, typically referred to as holes, move in from the anode. The positive and negative charges meet in the center layers (i.e., the semiconducting organic material), combine, and produce photons. The wave-length--and consequently the color--of the photons depends on the electronic properties of the organic material in which the photons are generated.
The color of light emitted from the OLED device can be controlled by the selection of the organic material. White light is produced by generating blue, red and green lights simultaneously. Specifically, the precisely color of light emitted by a particular structure can be controlled both by selection of the organic material, as well as by selection of dopants.
In a typical OLED, one of the electrodes is transparent. The cathode is typically constructed of a low work function material. The holes are typically injected from a high work function anode material into the organic material. Typically the devices operate with a DC bias of from 2 to 30 volts. The films may be formed by evaporation, spin coating or other appropriate polymer film-forming techniques, or chemical self-assembly. Thicknesses typically range from a few mono layers to about 1 to 2,000 angstroms.
OLEDs typically work best when operated in a current mode. The light output is much more stable for constant current drive than for a constant voltage drive. This is in contrast to many other display technologies, which are typically operated in a voltage mode. An active matrix display using OLED technology, therefore, requires a specific pixel architecture to provide for a current mode of operation.
The traditional approach to this problem is to use a pair of transistors and a storage capacitor. This allows for a quasi-DC drive of the OLED. This mode of operation increases the luminous efficiency of the device, while lowering peak current operating requirements.
This approach, however, presents several potential problems. First, for see-through applications, the non-emissive components of the device reduce the effective light emitting area. Given the relatively high efficiency of the OLED material, a low transconductance is typically required from the drive transistor M2, as shown in FIG. 1. This allows the drive transistor to be controlled via its gate source voltage with a practical range of approximately one volt. In practice, however, this translates to reducing the typical transconductance value of the device by five orders of magnitude. A reduction of this magnitude, however, is not easily achievable.
Second, the alternative of lowering the transconductance to acceptable values presents other challenges. The active material could be changed, for example, by replacing crystal silicon with amorphous silicon. This, however, results in impractical transistor characteristics for integrated drivers that are deposited on the same substrate as the emissive array.
Third, the current source formed by transistor M2 is controlled by only the gate voltage. This voltage is maintained by the storage capacitor, depicted as C1 in FIG. 1. For fixed level operation V.sub.DD, the only gray scale driving technique possible is current amplitude modulation. This presents unique challenges when operating the device in a gray scale mode with a large number of gray levels. The degree of modulation available becomes more tightly constrained, the larger the number of gray scale levels.
An alternative drive scheme is to pulse V.sub.DD once per frame. This allows the uniform scaling of the amplitude modulation according to the length of the V.sub.DD pulse.
Further, a reverse bias diode across transistor M2 may be required to refresh the OLED at the end of each frame. The conduction state of transistor M2 is data dependent and, therefore, unpredictable. Hence, the need for the reverse bias diode to reset the voltage prior to receipt of the next piece of data.
Although substantial progress has been made in the development of OLEDs to date, substantial additional challenges remain. For example, the whole class of organic devices continues to face a general series of problems associated with their long-term stability. In particular, the sublimed organic film may undergo recrystallization or other structural changes that adversely effect the emissive properties of the device.
Exposure to air and moisture presents unique problems with respect to OLEDs. Lifetimes of 5,000 to 35,000 hours have been obtained for evaporated films and greater than 5,000 hours for polymers. These values, however, are typically reported for room temperature operation in the absence of water and oxygen. Typically, the low work function cathode is susceptible to oxidation by either water or oxygen. Electroluminescence from these oxidized spots is typically darker than elsewhere. It is suspected that the oxidation induces delamination of the device.
Temperature stability poses additional constraints. Although the present inventors are aware of no published work at this time on the mechanisms for thermal degradation of these devices, the present inventors strongly suspect that thermal activation would effect the crystallization of the organic material as well as any reactions of the dopant and the organic materials making up the device.
Additional research has been done on understanding surface effects at the interface of the cathode with the OLED device. The present inventors also strongly suspect that the numerous reactions that can occur at the interface could strongly effect the lifetime of the device.
Prior inventors have attempted to solve the foregoing problems by using an alternating current drive scheme for the electroluminescent device. Tang, et al., U.S. Pat. No. 5,552,678 for AC Drive Scheme for Organic LED, discloses and claims applying an alternating voltage across the anode and cathode for driving an OLED. The use of alternating drive current improves the stability and operating performance of the OLED.
Most practical applications of OLEDs require limited voltage input, or light output, variance over an extended period of time. The limited stability of certain OLED devices has deterred, to some extent, their widespread use. When a constant voltage is applied, progressively lower current densities result. Lower current densities result in lower levels of light output with a constant applied voltage; eventually levels drop below acceptable levels.
If the applied voltage is progressively increased to compensate for this degradation and to hold light emission levels constant, the field across the OLED device increases. Furthermore, degradation may lead to a need for increased current to maintain constant light output. Eventually, a voltage level is required that cannot be supplied conveniently by the OLED device driver circuitry. Alternatively, the voltage level required produces a field exceeding the dielectric breakdown strength of the layers separating the electrodes. This results in catastrophic failure of the device.
Tang reports achieving stability and sustained operating performance of OLED by driving the device with an alternating voltage source. This alternating drive extends the useful life and reliability of the OLED, by eliminating catastrophic failure frequently encountered when the device is operated in a DC mode. With AC drive, the impedance of the EL device shows little variation with operation. Currents flowing through the OLED device with constantly alternating current do not significantly change with time. As a result, it is not necessary increase the voltage on the OLED to the same degree during operation. It is this increase that would eventually cause catastrophic failure due to dielectric breakdown of the organic film.
Tang also reports that the AC drive is crucial in eliminating shorting pathways due to located or point defects. These types of defects are often found in OLED and form semiconductive pathways between the anode and cathode. In operation these paths shunt current that would otherwise flow through the OLED film generating electroluminescence. This parasitic current loss is eliminated by the application of AC drive resulting in higher luminescence and operating stability.
Tang discloses that any AC drive scheme is better than a DC drive scheme in operating an OLED device. Tang asserts, however, that a certain form of asymmetric alternating current drive gives the best performance in sustaining the operational life of the device. Specifically, Tang discloses and claims an AC voltage cycle in which the reverse voltage of the AC cycle is less than the forward voltage. Specifically, in Tang, the voltage on the cathode is more positive than the voltage on the anode during the reverse portion of the cycle. During the forward portion of the cycle, the voltage on the anode is more positive than the voltage on the cathode. Tang also discloses that the time duration of the of the reverse portion can be significantly shorter than the time duration of the forward portion of the AC cycle. Tang claims that this asymmetry provides more time for the generation of electroluminescence, which is produced only in the forward portion of the AC cycle. Tang, however, does not disclose any particular structure for achieving AC drive.
FIG. 1 depicts a prior method of achieving AC drive. In particular, a pair of transistors and a storage capacitor may be used to achieve alternating current drive.
The present invention seeks to overcome certain of the problems that result from prior methods of obtaining AC drive and results in a design which is not only simpler in operation and easier to fabricate. The device of the present invention is also easier to drive than prior known approaches. The present inventors expect that it yield significant system cost reductions relative to prior driver methods.
First, the present invention is anticipated to reduce the size of the non-emissive driver devices, therefore, yielding more space on the emissive device, reducing the amount of light blocked by the driver circuitry and, effectively increasing the light emitting area of the device.
Second, the higher current requirement of the present invention reduces the need to lower the transconductance of the transistor.
Third, the present invention provides the ability to generate gray levels by pulse width modulation, in addition to amplitude modulation. This provides a more forgiving control mechanism and eliminates many of the challenges faced when operating on a gray scale mode width a large number of gray scale levels at low-end gray levels toward black. These levels typically require a very small difference between adjacent control levels. These levels can be generated using a pulse with modulation between two larger control levels. Further, the present invention eliminates the need for a reverse bias diode to refresh the OLED at the end of each frame period. The present invention forces all gates and all sources to ground, reversing the polarity of the other terminal of the OLED. This guarantees that all transistors are in conduction, allowing the use of a reverse bias cycle for the OLED. The conduction state is not data dependent and becomes more predictable.
The luminous efficiency of the OLED is reduced by operating at a pulse drive mode. Nonetheless, the advantages obtained by the present invention, in the present inventor's view, far outweigh this modest disadvantage.